(1) Field of the Invention
The present invention relates to a self-diagnosing apparatus and method of a variable valve timing structure for varying the opening/closing timing of an intake valve and/or an exhaust valve by changing the rotation phase of a cam shaft in an internal combustion engine.
(2) Related Art of the Invention
In the prior art, a variable valve timing structure for advancing or delaying the opening/closing timing of an intake valve and/or an exhaust valve by changing the rotation phase of a cam shaft in an internal combustion engine was known.
Further, when the rotation phase of the cam shaft was stable, abnormality of the rotation phase was diagnosed based on the detection results of the rotation phase (refer to Japanese Unexamined Patent Publication No.8-246820).
However, the diagnosis of abnormality in the prior art was for diagnosing the regular abnormality of the rotation phase, and it was not capable of diagnosing the deterioration of response characteristic of the change in rotation phase in the step-like change of the target.
When deterioration occurs to the response characteristic, it takes time to reach the required valve timing (rotation phase) from the driving condition, so the intake/exhaust valve could not be open/close driven by the most suitable valve timing during this response delay, which may deteriorate the driving performance. Therefore, it was necessary to diagnose the deterioration of the response characteristic.